The use of optical components in the industrial environment primarily requires intact and clean optical surfaces of these components. In particular in beam shaping and deflection of high-power laser beams, the important thing is to always test the characteristics of the optical components used, such as lenses and mirrors, in the context of their functionality, since the high laser beam power density on the optical surfaces of the components can quickly lead to contamination, destruction, and to consequential damage. Particularly critical are applications in the area of laser material processing, in which, as a result of the spatial proximity of the processing head of the material processing device to the workpiece to be processed, one must reckon with an increased contamination, for example, due to fumes and material spatters produced by the process.
In order to minimize the costs of the unavoidable contamination of the optical components, protective devices, such as cross jet and antireflective (AR)-coated protective screens, are foreseen, with which the functional optical components such as lenses and mirrors are protected from contamination. In this application, the main thing is the functionality of the aforementioned AR-coated protective windows.
Frequently, the contamination in a device for laser material processing, such as laser welding or laser cutting, arises in the following manner: Liquid metal spatters from the laser induced molten metal pool, reach the (process-side) surfaces of the optical component of the device, facing the processing point (TCP, tool center point). Some spatters adhere to the process-side surface of the optical component and are immediately burned into the surface by the high beam power density of the process laser beam. In this way, the transmissivity of the optical components is reduced in proportion to the surface of the burned-in spatters. At the same time, the burned-in spatters absorb the laser output, so that the optical components, which are not very good thermal conductors, are heated on one side (process side). Consequently, the optical component can sustain damage or even burst as a result of thermal stresses. In particular, there may be a deformation due to the heating of the optical components, which can have a negative influence on the position of the focal point of the process laser beam in the TCP, so that aside from the output loss due to the burned-in spatters, an additional damage of the process quality may occur.
From the state of the art, devices for the detection and monitoring of the contamination of optical components and, in particular, protective windows in order to avoid these effects in devices for laser material processing are known.
Passive monitoring methods use the detection of scattered radiation of the process laser beam that is scattered by the spatters formed, as in the case of the publications DE19605018-A1, DE19507401-A1, WO9833059-A1, DE19839930-C1, DE10113518-A1, and EP1488882-A1. This can be carried out, for example, with a protective window, on- or off-axis above the protective window (on the side of the protective window turned away from the process), or laterally, by the measurement of the scattered radiation, coupled into the protective window. The known methods, however, have considerable disadvantages, which greatly limit a practical application. For example, a desired change of the output power of the process laser leads to false alarm signals (with an increase in output) or to a loss of the monitoring function (with a diminution of the output, since the relatively small increase of the useful monitoring signal can lay under the alarm threshold). The influence of the radiation of the process laser, reflected from the workpiece, is also uncontrolled. Furthermore, this kind of device is not free of disturbances (“fail-safe”) in its mode of functioning, since in case of a breakage of the monitoring protective window or the not mounted or not properly mounted protective window, nothing can be measured.
Another known monitoring possibility is based on the measurement of the temperature of the protective window via contact temperature sensors. This takes place under the assumption that the burned-in spatters on the surface of the protective window lead to an increased absorption of the laser power and thus to a heating of the protective window. However, it has become evident that these known methods react in a very slow, not very sensitive manner and moreover, are dependent on the momentary value of the output power of the process laser and the laser power reflected from the surface of the protective window. Furthermore, these known methods are also not very free of disturbances in their mode of functioning since, in the case of a breakage of the monitored protective window or a not mounted or not properly mounted protective window, nothing can be measured.
A variant of the aforementioned passive monitoring possibilities provides for a measurement of the heat radiation of the burned-in contamination. It has become evident, however, that these methods are also dependent on the momentary value of the output power of the process laser and the laser power reflected from the surface of the protective window. Furthermore, this method is also not free from disturbances in its mode of functioning since, in the case of a breakage of the monitored protective window or a not mounted or not properly mounted protective window, nothing can be measured.
The active monitoring devices known from the state of the art, such as DE19839930-C1, DE20314918-A1, BE1007005-A1, EP01398612-A1, and DE19654850-A1, use additional light sources (for example, LEDs or laser diodes), in order to detect certain features of the protective window. Usually, several additional light sources are used with different functions, so as to guarantee a comprehensive monitoring. Thus, for example, a breakage of the protective window can be reliably detected. Alternatively, the active methods are also supplemented by passive measurement methods, which, however, have the disadvantages already indicated above and, moreover, increase the complexity of the monitoring device.
An active monitoring device known from BE 1007005-A1 and EP 01398612-A1 uses a type of reflected light barrier. With this monitoring device, both the contamination as well as a breakage of the protective window can be, advantageously, quickly detected and this can be largely done independent of the laser output power of the process laser. Here also, the proposed solution is not optimal, since the sensitivity of the method is very low and greatly dependent on the optical characteristics of the scanned surface of the protective window. Furthermore, the measurement is also greatly dependent on the output power of an additional light source that emits a measurement beam onto the surface of the protective window.
A detection and monitoring method for the contamination of an optical component is known from EP 1354664-A1, where a measurement beam from a light source is projected under an incidence angle onto the outer surface of the optical component. Here in a first example of the set-up a light sensitive detector measures the scattered light from the outer surface of the optical component. In a second example of the set-up a detector measures the intensity of a beam, which is reflected from the outer surface of the optical component.
Another method for the detection and monitoring of the contamination of an optical component is known from DE 10 2004 041682-A1. Here the measurement radiation of a light source is projected onto a surface of an optical component under an incidence angle and the reflected and scattered radiation from the surface of the optical component is measured with a light-sensitive detector.